Referring to FIG. 1, a conventional aircraft is generally indicated at 2. The aircraft 2 comprises a fuselage 4 and a pair of main engines in the form of main gas turbine engines 10 mounted on wings 6. The aircraft 2 also includes an auxiliary power unit (APU) 8, mounted in the rear of the aircraft fuselage 4.
FIG. 2 shows one of the main gas turbine engines 10 in more detail. Each main engine 10 comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19. The gas turbine 10 further comprises low pressure 38, intermediate pressure 39 and high pressure 40 shafts, which mechanically couple the fan 12, intermediate pressure compressor 13 and high pressure compressor 14 to the low pressure 18, intermediate pressure 17 and high pressure 16 turbines respectively. The APU 8 also conventionally comprises a gas turbine engine, generally comprising a combustor, and a single compressor and turbine, driven by a common shaft.
As well as providing thrust from the propulsive airflows A and B, each engine 10 also provides electrical power for various engine accessories, such as fuel and oil pumps, and for aircraft subsystems such as environmental control, avionics and electrical actuators.
FIG. 3 shows a prior aircraft electrical network 20. The network 20 comprises a plurality of electrical generators 26 which provide alternating current (AC) electrical power to a main AC electrical bus 28. The generators 26 are mechanically driven by a pair of main gas turbine engines 22, an auxiliary power unit (APU) in the form of a further gas turbine engine 24, and a Ram Air Turbine (RAT) 25. Typically, two generators 26 are driven by the same shaft of each main engine 22. The electrical network 20 further comprises a pair of Direct Current (DC) engine electrical buses 30. Each DC engine electrical bus 30 is powered by a one-way AC to DC power converter 32, generally in the form of an auto transformer rectifier, which is in turn powered by the main AC bus 28. Each DC electrical bus 30 powers a plurality of electrical components such as motor controllers 34. The motor controllers 34 are one-way DC to AC converters which in turn power respective AC electrical motors 35 used to drive aircraft electrical loads such as cabin air compressors.
In order to start the main engines 22, one of the generators 26 is operated as a motor by supplying AC electrical power to the generator 26. The AC electrical power is provided by one or more of the motor controllers 34 via an AC interconnector 36, which provides AC power to the starter generator 26 from the motor controller 34. Once the main engines 22 are started, this interconnector 36 is no longer used. In view of the high power requirements of the starter generators 26, and the relatively long cable run of the interconnector 36 between the motor controller 34 and main engines 22, the interconnector 36 is relatively heavy, weighing approximately 40 kg in the case of the interconnector on typical wide body aircraft. It is therefore desirable to provide AC electrical power to the starter generators 26 without the requirement for a heavy AC interconnector connecting the motor controllers 34 and main engine starter generators 26. This method of providing electrical power to the main engines 26 also does not provide any redundancy, since if the interconnector 36 is not available, no power can be transferred to the starter generators 26.
In a separate problem, in the case of civil aircraft, it is generally necessary to be able to recover from a mid-air main engine shutdown by restarting the main engine 22. This may be done using the main engine starter generator 26, powered either by the APU or the other main engine's power offtake, by “windmilling” the shutdown main engine, i.e. by allowing air flow through the engine caused by the forward speed of the aircraft to turn the engine, or by a combination of these methods. In any case, it is necessary for the high pressure compressor 14 and turbine 16 to operate during mid-air restarts in order to provide sufficient pressure in the combustor 15 to support combustion. Where the main engine starter generator 26 is powered by the APU 8, there is necessarily a delay while the APU 8 is started, which may be unacceptable.
Furthermore, where the high pressure shaft 40 is used to operate an electrical generator, a large electrical offtake from the high pressure shaft 40 can impact on engine operability during off-design periods (such as at flight idle). During these periods, it may be necessary to open bleed valves (not shown) in order to prevent stall or surge of the compressor 14, which results in low fuel efficiency (i.e. higher fuel burn).
The present invention describes an aircraft electrical system which seeks to overcome some or all of the above problems.